Various types of optical displays are commonly used in a wide variety of applications. For example, computing devices such as personal computers, workstations, and personal digital assistants (PDA) and communication devices such as mobile phones and radios all use various types of displays. Displays are also commonly used for a variety of purposes on vehicles such as automobiles and aircraft. Optical displays can use a variety of different display mechanisms, such as LCD, CRT, projection and other devices.
One new application in which optical displays are being applied is in aircraft avionics systems. For example, in an aircraft avionics application, a multi-function display can be used to provide a variety of different types of information to the pilot by dividing the display into a plurality of windows, and assigning each window a display task. For example, in a multi-function display one window can be assigned to display primary flight control information such as airspeed, attitude, altitude and horizontal situation. Another window can be used to provide navigation information such as heading and location. Another window can be used to provide information such as electrical, hydraulic, or cabin pressure values or status.
One important performance parameter in certain displays is the range of luminance that can be provided by a projection display, commonly referred to as the dimming range. In many applications it is critical that a display make information clearly visible in a wide variety of ambient light conditions. For example, a display used in an aircraft avionics system will need to display information to the pilot under lighting conditions that can range from near total blackness to the extreme glare created by facing directly into daytime sunlight. Such a display must be able to display images at a wide range of intensities. Without a sufficiently large range of intensities a viewer of the display may be unable to easily read information from the display in high ambient light conditions, low ambient light conditions, or both.
One important characteristic of certain images is the selection of colors to convey specific meaning, such as blue indicating water. For such images, it is desirable to leave the hue unchanged when adjusting displayed image brightness.
The content of the images displayed can have negative impacts on the ability to adjust the intensity of the displayed image without adversely affecting image quality. For example, when attempting to display an image that includes a large percentage of white or other light colors, the resulting bright light can degrade the pilot's night vision adaptation. Attempts to reduce the intensity by dimming the image can cause a significant loss of image quality. Specifically, uniformly dimming an image can cause a significant loss of contrast in the displayed image, causing elements of similar color and contrast to lose significantly readability. Furthermore, attempts to limit brightness of one window by dimming the entire display can negatively impact the view ability of other windows on the display.
Thus, what is needed is a system and method for altering a displayed image to reduce the overall intensity while maintaining good contrast and preserving the hues of the original image to the extent possible.